The isolation, characterization, and physiological function of transglutaminases that catalyze the formation of covalent crosslinks between protein molecules are being studied. Three distinct classes of these enzymes in guinea pig and human tissues and organs have been defined in this laboratory. Similar studies are underway in other species including Limulus polyphemus, Humarus Americanus and Petromyzon Marinus. The enzymes of these species participate in haemostatis directly without limited proteolytic activation of a zymogen form. The presence of high concentrations of a transglutaminase in red blood cells of human and guinea pig is under further study. Of particular interest are the roles of the individual transglutaminases in blood coagulation, $ wound healing, seminal plasma coagulation during fertilization, cell membrane formation, crosslinking of hair proteins and epidermal keratin formation. The biosynthesis, degradation and hormonal regulations of plasma protransglutaminase are also under investigation.